1. Field of the Invention
This invention relates to in-orbit testing of communication satellites, and more particularly to in-orbit receive antenna pattern testing from a single location on the ground of a satellite having multiple receive antenna beams covering a large geographic area.
2. Brief Description of the Related Art
After a successful launch, it is essential to test a satellite's subsystems while it is in orbit and compare the results with prelaunch data to verify that the satellite is compliant with customer specifications. Testing of the transmit function in orbit allows the ground station to map the satellite's area of coverage to compare against coverage requirements for two reasons. First, to determine that the satellite is operational and has not been damaged during launch, and second, to verify that the satellite and its antennas are oriented so that each downlink beam area of coverage meets customer specifications.
A typical communications satellite consists of one or more uplink receive antennas for reception of signals, one or more downlink transmit antennas for return transmission, and complex electronics, referred to as a repeater, that receive and amplify the uplink signals before relaying them back to receiving stations on earth. Ground test stations are set up for execution of the in-orbit testing, which includes measuring and mapping the satellite's receive antenna performance to verify its integrity.
A brief history of in-orbit test procedures and facilities is described in Kasstan et al., "Recent Advances in `In-Orbit Testing` of Communications Satellites", International Journal of Satellite Communications, Vol. 13, No. 5, September-October 1995, pages 377-401. Improvements to in-orbit testing over recent years have been in the area of equipment automation using computers; actual test procedures have remained basically the same. A ground station antenna forms the necessary interface to the satellite and the host computer maps the pattern of reception based on the uplink signal transmitted to and the downlink signal received from the satellite.
The receive antenna mapping procedure traditionally used for simple antenna systems was derived from the transmit antenna test. Basically, the transmit antenna test is performed using the satellite's on-board automatic level control (ALC) if available to level and amplify the uplink test signal, or by driving a non-linear high power amplifier slightly past saturation to provide a relatively constant downlink signal to produce a transmit pattern map. Then the test is repeated in a linear mode below saturation without the ALC function to generate a combined transmit+receive map. Subtracting the transmit component from the transmit+receive map results in a receive antenna pattern. The test is repeated for each receive antenna. For this test to work, the transmit and receive patterns must be similar in coverage.
Recent advances in satellite communications have resulted in satellites that have an increased number of smaller receive uplink and transmit downlink beams. Receive antenna beams allow uplink sites to transmit information from areas within the satellite's coverage. Increasing the number of receive antenna beams allows the customer to originate different information from various target areas with the potential of re-using the same frequency spectrum for the non-overlapping reception areas of different receive antennas. As the number of receive antennas and downlink beams increase, so must the complexity of the satellite circuitry, resulting in an increased number of in-orbit tests to be performed. For a design with uplink beams thousands of miles apart, a separate ground test station is required to test the satellite performance from each uplink region. To measure the downlink signal returned, multiple test sites may also be required. An uplink test signal is generated and transmitted from an uplink site. Downlink test sites within the areas covered by the downlink beams record the power level of the signals returned. The downlink test stations are often mobile stations that are able to measure signals returned in remote areas. The antenna pattern is then mapped by correlating the power of the signal returned with the location of the ground test station receiving it.
The test procedure for testing satellites with multiple receive beams is basically the same as for testing a satellite with only one receive beam. The primary difference is that, when testing multiple receive beams, the test procedure is repeated for each uplink antenna beam from a pair of ground test stations, one located in the coverage area of the receive antenna beam providing the uplink, and the other located within the coverage area of the antenna transmit beam. This increases the number of ground test stations required. In addition to being arduous and time consuming, it requires numerous ground test stations and qualified in-orbit testing experts.